1. Field of the Invention
The present invention relates to the art of well drilling and earth boring. More particularly, the invention relates to methods and apparatus for perforating wellbore casing or casing liner.
2. Description of Related Art
After the actual drilling of a borehole into the earth, the shaft is often prepared for long term fluid production by a series of steps and procedures that are collectively characterized by the art as xe2x80x9ccompletion.xe2x80x9d Among these numerous procedures is the process of setting a casing, usually steel, within the borehole to line the shaft wall with a stable, permanent barrier. This casement is often secured by cement that is pumped into the annulus between the outside diameter of the casing and the inside diameter of the raw shaft wall.
While the casing stabilizes the shaft wall, it also seals the fluids within the earth strata that have been penetrated by the borehole from flowing into the borehole. The borehole inflow of some of the fluids is the desired objective of making the borehole in the first place. To selectively open the casing to such fluid flow, the casing wall is often penetrated in the region of a production zone by shaped charge explosives or xe2x80x9cbulletsxe2x80x9d. Numerous charges or bullets are loaded into tubular xe2x80x9cgunsxe2x80x9d, usually in a helical pattern along and around the gun tube axis for positioning within the wellbore at the desired location. The line of discharge from the gun is radial from the gun tube axis.
The downhole environment of a deep earth boring is frequently hostile to the extreme. The borehole is usually filled with a mixture of drilling fluids, water and crude petroleum. At such depths, the bottom hole pressures may be in the order of tens of thousands of pounds per square inch and at hundreds of degrees Celsius temperature. Consequently, by the time the perforating gun arrives at the desired perforation location, the ignition system, the explosives or the propellant charges are sometimes compromised to the extent that discharge fails to occur on command. In anticipation of such contingencies, provision is often made for unrelated alternative firing systems. If all else fails, the defective gun must be withdrawn from the well and repaired or replaced and returned.
As a further consideration, many of the well completion steps require specific tools that are operatively secured within the length of a pipe or tubing work string and deposited into the wellbore from the surface. Placement of a completion tool on downhole location may require many hours of extremely expensive rig time and skilled labor. The full cycle of down hole tool placement and return is termed in the art as xe2x80x9ca trip.xe2x80x9d
At the present state of art, many of the necessary well completion tools are assembled collectively on a single work string and run into the wellbore together for the purpose of accomplishing as many of the several completion steps in as few xe2x80x9ctripsxe2x80x9d as possible. There could be many advantages, therefore, for including the perforation gun at the end of a completion tube. In a single trip, the well could be perforated, fractured, packed and produced. On the negative side, however, should the gun misfire, it would be necessary to withdraw the entire work string to repair or replace the perforation gun.
Comparatively, tools and instruments suspended from drum reeled xe2x80x9cwirelinesxe2x80x9d are run into and out of a wellbore quickly and efficiently. It would be advantageous, therefore, to position, secure, remove and/or replace a perforation gun or other such tool entirely by wireline.
Some completion assemblies connect the gun to the work string in such a manner that releases the spent gun tube to free fall further down the wellbore below the perforated production zone. In some cases, this gun release function may be desirable. In other cases, especially when additional drilling may be contemplated, the spent gun becomes downhole xe2x80x9cjunkxe2x80x9d and must be extracted by a fishing operation.
It is, therefore, an object of the present invention to provide a means and method for securing a perforating gun to the end of a completion or production tube for alternative operational modes. In one mode, the gun may automatically disconnect from the work string when the gun is discharged and free fall from the perforation zone. In another operational mode, the gun may be tethered to a wireline and withdrawn from the well after discharge.
Another object of the invention is provision of a perforation gun assembly that may be lowered into a well along a work string tube bore at the end of a wire line, secured to the tube bore at the desired position and discharged. In the event of malfunction, the gun may, by wireline, be disconnected from the work string tube and withdrawn for repair.
As an initial description of physical relationships, the perforation gun and its associated tubing connection module are sized to pass internally through the bore of a tubing string suspended within a well bore. Such tubing around the gun may be any number of working string elements such as the tail pipe of a completion string or a production tube for example. Within this control parameter, the connection module preferably comprises two expandable dog connecting mechanisms. The first set of connecting dogs secures the perforating gun to the connection module whereas the second set secures the connection module to the bottom end of the work string tubing.
The first or lower set of connecting dogs are released by gas pressure generated by the perforation propellant. When the gun discharges, propellant gases generate a pressure surge within the bore of the perforating gun which are channeled to act upon one annular end face of a sleeve piston. The sleeve piston is thereby displaced by a resulting pressure differential to align a reduced radius release perimeter along the piston surface under the first dog set. When the release perimeter is aligned with the first connecting dogs, the dogs radially retract from a position of meshed engagement with a circumferential ledge that is formed around the inside perimeter of a cylindrical counterbore in the connection module socket cylinder. Upon radial retraction of the first connecting dogs, the spent gun is free to axially slide along the connection module socket cylinder for a limited distance.
The second or upper set of connecting dogs are expanded into a circumferential latch channel formed around the inside bore of the work string tube. Radially shifting latch pins are caged by a setting piston and externally meshed with a latching cone. Internally, the latch pins are supported by a surface profiled latch tube. A connective relationship between the work string tube and the upper connecting dogs is maintained by shear pins and screws through the upper latch profile tube and the upper latch setting piston.
When the spent gun shifts downwardly, the profiled upper latch tube is pulled down to shear the respective retaining pin and remove the radial support structure under the upper latch pins. Without interior support, the upper latch pins retract radially inward to release the upper connecting dogs from the work string latching channel. When the upper connecting dogs retract from the work string latching channel, the connection module and spent perforating gun are free to fall away from the end of the work string tubing.
In an alternative operational mode, such as when the gun fails to discharge, the upper connecting dogs may be retracted by a wireline pull on the upper latch profile tube. This releases the gun and connection module assembly as a unit from the work string tube. At any time, the unit may be drawn out of the wellbore at the end of the wireline along the work string internal bore, replaced or repaired and returned.